Switching Genes On and Off

Teacher Resource

Posted 03.10.10

NOVA

In this video segment adapted from NOVA, evolutionary scientist Sean Carroll investigates why one species of fruit fly has wing spots while another genetically similar one does not. As the video explains, although two organisms may both possess the same gene linked to a particular trait, this trait may not be expressed unless the gene is turned on. This is done by an enhancer, a piece of noncoding DNA—previously thought of as "junk" DNA—that acts as a "switch."

Background

The new science of evolutionary developmental biology, or "evo devo," applies lessons from the development of individual organisms to a broader understanding of changes in groups over the course of evolution. By studying developing organisms, scientists have learned that certain regions of DNA that were long believed to have no function actually regulate gene expression by controlling where, when, and even to what degree a particular gene is turned on.

All living things get their building and operating instructions from DNA. Coiled inside the nucleus of the cell, DNA stores all the information needed for an individual to develop and function as a living organism. The information is packaged in genes, the units of heredity, which are distributed along the chromosomes of an organism. Each gene contains a coded instruction for making a single protein, and it is proteins that influence the collection of traits an organism has.

The protein-making instructions of DNA come from short sections called coding regions, or exons. The term "junk" DNA was originally coined to refer to regions of DNA on either side of the coding regions that seemed to contain no genetic information. However, scientists have recently discovered that these "junk" regions actually contain molecular mechanisms called gene switches that govern whether, when, and in some cases how much of a gene is activated.

Gene switches are the binding sites for regulatory molecules. These are proteins that, when they land on a gene switch, trigger transcription and thus express the gene. Transcription is the process whereby the DNA sequence in a gene is copied into mRNA and ultimately translated into a protein.

The video examines two related fruit fly species (Drosophila biarmipes and D. melanogaster), both of which have a so-called paintbrush gene responsible for coloration in the flies. Both species have switches that govern how the paintbrush gene is used. For example, one switch along the noncoding DNA region colors the abdomen of both flies in the same manner. However, D. biarmipes has an additional switch that controls expression of the paintbrush gene in cells in the wings as well. As the fruit flies demonstrate, the same gene can produce very different traits in two species. One species uses the paintbrush gene to make wing spots, while the other doesn't. It all depends on how the gene is regulated during development.

Sean Carroll, who is featured in this video, hypothesizes that evolution happens not by making new genes, but because of mutation to switches that do new things with existing genes. When a gene switch is added or deleted, new variations result. As new traits get passed on to offspring, new species are eventually formed.